Here in MD I was not impressed much with what they were turning out from the MSF classes based upon the skills from two of my buddies who are new to riding and just graduated. I didn't take the MSF here in MD since I got my license in Ohio back in 1986. The MSF is better than nothing, but it didn't do much for a couple guys I know other than teach them to go around a parking lot. I often recommend the Total Control Clinic (by Lee parks) that is often give at MSF locations on alternate weekends---it's more about cornering and braking. Here in MD they don't do much work with skid control from what I understand---but of course it's better than having nothing.
From riding on the track, and even some aggressive street riding let's say...I've locked up a tire or two on my Ducati and R1
You are right....if you skid the back tire it is best to stay on the brakes until you come to a stop...unless you know what you are doing. The danger is the obvious highside when the rear tire regains traction. It can happen to even the best of pro riders on the track. My buddy did it on his 999 right in front of me running at VIR. He crashed and did $3600 damage to his bike.
The only time that I will let off the brakes in a skid is if I can somehow get the bike skidding straight with both wheels in somewhat of an alignment. If the back tire is out to the side and you let off the brakes you're probably going to get bucked off. It is really a case by case thing. As you know